Power conversion system and apparatus



E1s. TRUE POWER CONVERSION SYSTEM AND APPARATUS Aug.4 23, 1949.

4 Sheets-Sheet 1 Filed Aug. 25, 1946 INVENTOR.

Aug. 23, 1949. E s, TRUE 2,479,705

POWER CONVERSION SYSTEM AND APPARATUS Filed Aug. 23, 194e 4 sheets-sheet 2 a 57 f5 f7 ,96' f4 W55 '2 SUAP-7%. 55 Z4 i "ml i 5.2 7a

INVENTOR.

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z/wf/ @da Aug. 23, 1949. 5 5, TRUE POWER CONVERSION SYSTEM AND APARATUS 4 Sheets-Sheet I5 Filed Aug. 23, 1946 d e ww M 7% um@ w a a m7 d N @Q d w 7 W W j N w, w 3 1 M 5 5 M Il l mw 6 3 a 7 9 .a J W3 Ov 3 7 Z 9 9, J a f, if MV Z v U d 3 2 e J a J 9 s n 5 i d ad J o @D a f a W ,W/ uw 5 O 5 0 a a g, ma 7 l M07/ 4 5 l d 6 J 0 J 7 W N Aug. z3, 1949. E. s. TRUE 2,479,705

POWER CONVERSION SYSTEM AND APPARATUS Filed Aug. 25, 1946 4 Sheets-Sheet 4 Patented Aug. 23, 1949 POWER CONVERSION SYSTEM AND APPARATUS Edwin S. True, Oak Park, Ill., assigner of onehalf to Joseph Waitcus, Chicago, Ill.

Application August 23, 1946, Serial No. 692,672

(Cl. S- 15) 17 Claims.

This invention relates to welding systems and apparatus oi the type suited to electrical welding and particularly to electrical spot welding.

One of the more general objects of my present invention is to provide an electrical welding system and apparatus adapted to a wide variety of .uses and which may be utilized in most localities and locations without making any special provisions for suiiicient source of power.

Another general object of my invention is to provide a welding system and apparatus suited to spot welding and which are designed to avoid excessively high peak power demands and poor load factor.

My present invention -further comprehends the provision of a system and apparatus for electrical spot welding which not only has the advantages which have been heretofore mentioned, but which is also readily adjustable to suit a variety of jobs and uses.

As another object my invention contemplates the provision of a compact and durable power conversion unit for use in electrical spot welding, which unit is adapted to furnish preselected 'high values of welding current at intermittent and relatively short periods and to receive electrical energy from a power source at a relatively even rate during and between the welding periods. Y

In Iorder to effect ease of adjustment of my power conversion unit to suit it to the effective welding of different kinds and weights of materials, it is an object of my invention to provide an electrical system embodying a secondary power source and a current dividing circuit including a readily variable resistance unit so connected that it controls the maximum and instantaneous values of welding current during each welding operation.

My invention additionally includes the provision of an electrical system for spot welding which embodies a current dividing circuit incorporating a plurality ci secondary power sources of diii'erent sizes so connected that they receive electrical energy from a common source, although the circuit may be varied to alter amount and rate at which each of said sources contributes to the system for welding purposes, whereby the welding current value and power may be selected to suit a given job.

As another object the invention contemplates the provision of a welding system wherein secondary batteries are .used for stored energy power conversion, and a unique system of relatively small water-cooled conductors serves to 2 establish the electrical connections between the batteries as well as the cells thereof, and to maintain a uniform low temperature and correspondingly uniform resistance values.

It is to be understood that the term welding as used herein, is to be interpreted in a broad sense as applied to all forms oi bonding metals, including processes sometimes referred to as soldering brazing or plating Other objects and advantages of the invention will be apparent from the following description and the accompanying drawings in which similar characters of reference indicate similar parts throughout the several views.

Referring to the drawings, of which there are four sheets:

Fig. 1 is a perspective View taken from in front and to one side of a unitary power conversion unit adapted to use in electric welding and embodying a preferred form of my present invention.

Fig. 2 s a perspective view of the power conversion unit shown in Fig. 1 taken from the rear and toward the side opposite that shown in Fig. 1 and wherein most of the outer housing is removed to show the internal structure of the unit;

Fig. 3 is a fragmentary perspective View looking downwardly and toward one side of the power conversion unit shown in Figs. 1 and 2;

Fig. 4 is a perspective side View of a. portion of the apparatus depicted in Figs. 2 and 3;

Fig. 5 is a schematic circuit diagram which illustrates my preferred electrical connections for the Welding system and apparatus disclosed herein;

Fig. 6 is a schematic circuit diagram similar to Fig. 5, but illustrating a modication of my invention; and

Fig. '7 is a fragmentary front elevational view, drawn to an enlarged scale, and depicting a modification of the structiu'e shown in Fig. 4.

Considered generally, my preferred welding system and apparatus embodies a unitary power conversion unit having a plurality of storage batteries and a battery charging unit connected in a circuit such that each contributes to the supply of relatively high values of direct current at comparatively low voltage to suitable welding electrodes. The current output capacity of the battery charging unit is such that it contributes an appreciable amount of current to the welding electrodes during welding operations and charges the storage batteries during the periods between such welding operations. For flexibility in use in the welding of diierent kinds and weights of materials, I prefer to divide the storage batteries into groups having substantially equal voltage ratings and different current :output capacities and to interconnect the storage batteries and the battery charging unit through a rheostat in such way that the proportionate amounts contributed by each of the battery groups and the battery charging uni-t may beadjusted to effect the de,- sired current output characteristics. Such variation of the current output characteristics supplements variations which are accomplished through the timing of the welding.. .operation either manually or by an automatic timing device. With my preferred `circuit connections the welding electrodes may also' be connected at different places to the power conversion' unit to effect an additional variation .of 7the Youtput. current characteristics.

Referring more particularly to the preferred embodiment of my invention which is depicted in the accompanying drawings for exemplary purposes, I -have shownl in Figs. 1 and 2 a unitary power conversion assembly which is adapted to be Yconnectedtoa suitable commercial source of alternating current and to provide high values of direct current at rela-tively 4low voltage to any desired size and type ofV welding electrodes. In my presently disclosed uernbociiment the power conversion unit is carried 4by a supporting structure including a base- |12 lhaving upright support posts i3, lll, l5 and I5 secured to the corners thereof and carrying a shelf structure Il' at a position above the base and below the tops 0f the support posts. By preference, casters i8 support the base so that the unit may ibe readily moved from place vbase l2, and, inthe present instance, is secured in place by a metallic strap 26 which extends over .the top of the unit,.as shown in Fig. 2, and is secured. at its ends to the base by fastening means,

such as bolts and co-actingwing .nuts 2'! .(Fig. 1v). -This `batter/y charging `unit-may be any one of a variety .of types which usually incorporate a .transformer and a rectifier unit for converting an available alternating current to a direct current at .suitable voltage for chargingthe batteries at a desired rate. The alternating currentis connected to the battery charging unit .through an ,adapterV 28 (Fig. l) which is accessible through an opening 29 in one of the housingend .walls 20,

For the present purpose the output rating of the battery charging unit is desirably such that it will not only charge the batteries during the periods between welding operations but will also contribute an appreciable amount to the vwelding current during the welding operations. For power conversion units of the size contemplated, they Vbattery charging .unit is provided with an electri- 'cally driven fan 35 for circulating cooling air .across the operating parts of the -battery charging unitwhile it is in operation. To facilitate the flow of this cooling air, I have provided openings In the `present instance the batteries 35, and

37 are similar and each includes a plurality of cells 39, @il and i2 as shown in Fig. 3. Each such cell has a pair `of output terminals t3 and lill.

As 'previously mentioned, and for the purpose of lending; flexibility to the output current characteristics which are available from my power conversion unit for a desired purpose, I prefer, in the electrical connections of the cells to divide those cells or batteries intogroups, which groups have substantially the-same output voltage rating but different output current characteristics. IjIaving reference to Figs; 3 and 5, wherein the circuit connections of the batteries are depicted in their physical form and schematically, the three batteries which are utilized in the present instance have one common terminal connector which serves as one of the output terminals of the v oltage conversion unit and is connected to terminal posts 63 of like polarity on each ofthe batteries V35, 36 and Si' by a conductor d5. One of the output leads i? of the battery charging unit 25 is also connected to that terminal connector 45 through a switch lis and a lead i9 clamped onto the conductor 6:5.

The cells of the batteries 35 and 35 are connected in series-parallel circuit relationship, while the cells of the battery 3l are connected in series circuit relationship. Since the number of cells in each of the .batteries 35 and 36 which are connectedin'series-parallel relationship is equal to the number of cells in the battery 3l, the'volt- Aage ratings of the two groups of batteries thus connected are substantially equal. However, the series-parallel connection of the cells of the batteries 35 and 35 provides a greater output current capacity than that of the single battery 3l having its cells connected in series.

The other end terminal of the battery 3l is con nected by a conductor 5st to one end of a resistance leg 52 of a rheostat 53, while the other end terminals of the lseries-parallel connected batteries 35 and 36 are connected through a conductor 513 to a second resistance leg 550i the said rheostat. The other en ds of the resistance legs 52 and 55 are electrically connected together and are provided with a terminal connector 55 which serves as a fixed mid-tap on the rheostat to which the other output lead 5l of the battery charging unit is connected. It may therefore be understood that the battery 3l and the series-parallel connected batteries 35 and 36 are effectively connected in parallel relationship with the battery charging unit through the resistance legs 52 and 55 respectively of the rheostat 53.

As shown in Fig. 4, the resistance legs 52 and V55 of the rheostat 53 are disposed in substantially parallel relationship to one another so that a movable clamp-type contacter 58 may be manually moved to various positions along the resistance legs and secured in any desired position .by fastening means such as screws 59. Since the resistance legs 5 2 and 55 are connected together -at one end, the movement of the contacter 53 Vpipe 63.v

along the resistance legs changesthe proportionate amounts of the legs which are connected inparallel so as thereby to vary the resistance values in the circuit between the batteries and between each of the batteries andthe battery charging unit. i

Since the values of current utilizedin welding are relatively high, the lactual resistances of the legs 52 and 55 may be relatively smallffor effect- Ving substantial control but they must have high lcurrent carrying capacity and should remain substantially constant in resistance for consist- Avency offoperation. With these factors in mind,

`insulating blocks 68 and 69,' respectively, which areeach secured to one of the posts and have the ends of the resistance legs secured thereto by fastening means, such as U -bolts 19.

In order to counteract the heating effect of the current flow through the rheostat and thereby to maintain the resistances of the parts thereof at substantially uniform valuesduring operation, I prefer to provide a flow of coolant through the tubular legs thereof. For effecting such a coolant ow,'I have provided couplings 92 and |83 which communicate with the resistance legs 52 and 55 through the fittings 64 and 65 so that Acoolant may ow from one coupling to the other through the resistance legs and the connecting In addition to cooling the rheostat 53, I also vprefer to cool the conductors which interconnect the cells of the batteries and to do this in a `manner such that both the rheostat and the connecting conductors for the batteries are cooled by a single continuous ow of coolant. A single conductor 12 interconnects terminals 44 of the cells 39 and vterminals 43 of the cells A similar conductor 13 inminals 43 of cells 42 on those same batteries,

'While the conductor 54 connects the terminals 44 ofthe cells 42 in addition to connecting those terminals to the resistance leg 55 of the rheostat.

`On the battery 31 the cells are connected in series by conductors 14 and 15. `The conductors 46, 12, 13, 54, 50, 15 and. 14 are all tubular and the'connections of the conductors 54 and 50 to the legs of the rheostat are such that they provide iiuid communication as well as electric-al connections tov those legs. NVSince the voltage of the batteries and particularly-the voltages betweencells is low, I have round it entirely practical to interconnect .the tubular conductorslfor the batteries by flexible vinsulating tubes o rihoses which are in each in- .stance clamped to the ends of the conductorsfor ,fluid-tight connections thereto; that is',.one end ofthe conductor y46 is connected to one end of theconductor 12 by a hose 16.Y The other end of the conductor 12 is connected to one end of the conducton13 bya hose 11. The other end of .the conductor '13-i s connected tothe end of the conductorv 54v vby a, hose 18. On the battery 31 .-thewerid of theconductor50 is connected to -one end of the lconductor `15 by a hose 19 and the other end of the conductor15 is connected to one end ofthe conductor 14 by ahose 80. The other end of the conductor 14 is extended asindicated at 82 in Figs. 2 and 3 to provide one fluid ow connection to an end of the series of con.- ductors and hoses. The otherend connection is indicated at 83 in Fig. 2 and is provided by a tting on the terminal post 45 to which the conductor 46 is connected. Thus it may be understood that a coolant such as water flows into one end of the connected system of conductors and hoses, as at the fittings 83, and from there flows through the tubular conductors, hoses, and rheostat legs in the following sequence: 46, 18, 12, 11, 13, 18, 54, 55, 52, 5D, 19, 15, 80, 14, and thence to the tting extension 82.

For convenience in adjustment during the use of the power conversion unit, the rheostat 53 is mounted along one side thereof and the screws 59, by which `the movable contacter 58 is llocked in place or freed for movement, extend outwardly through an opening 84 in one side wall of the outer housing, as shown in Fig. 1. For convenience in resetting the movable contactor .to` a predetermined position, a scale 85'may be attached to the outer surface ofthe housing-side wall adjacent the opening 84. 'f

In the disclosed embodiment of Iny invention I have also provided for the visual indication of the operation of the power conversion unitby meters 86 and 81 mounted on a panel 88 at the top of one end wall 20 of theouter housing and exposed through an opening 89 therein. The meter 86 provides an indication of the battery cell voltage of one pilot or indicator cell 42 in battery 35. Meter 81 provides an indication of either the charging rate or current output ofthe battery charging unit 25, or the total current output of the power conversionunit. As is quite common in the measurement of currents of the magnitude encountered in apparatus of the type disclosed, I utilize a millivoltmeter for measuring current values by connecting thermillivoltmeter 81 across a calibrated resistor in the circuit. In the instance, of measuring the total current output of the power conversion unit, a section 9o (Fig. s) of the conductor as 'is anvbrated and utilized for the resistor across which the meter 81 is connected. A pair of insulated conductors 92 hasends 93 and 94 connected to Vthe conductor 46 at opposite ends of the calibrated section and affords the connection to the meter 81 through a double pole double throw toggle switch 95. In like manner the meter 81 may be connected across a. section of the output lead 51 through the toggle switch 95. By loperating toggle switch 95 to one position or the other, the rectifier output or the total output is measured. By operating toggle switch 96 the meter 86 may be made to indicate the pilot cell voltage with the charger on or offj as desired. The sections of the conductors 46 and 51 across which the meter 81 may be connected by operation of switch 95 are indicated as resistors Rl and R2 respectively in the circuit diagram of Fig. 5.

In any instance during use of the power conversion unit, one welding electrode 91 (Fig. 5) is preferably connected tothe common terminal connector 45. However, the other Welding electrode 98 may be connected to different parts 'of the circuit to obtain variations in the welding current characteristics. As shown in solid lines 75 nl'fig. 5 the electrode,.98 is .connected tothe tenute-e1 post-eri at the-end# ertnefres'istanceieg 55.; Alternative electrode'l conneotionslare -indir'- cated nime circuit diagram" 1mg.' 5 in dot and dash lines.- Inleacliiiistanr'ze, 'ls'ipreviously mentioned-Q one electrode is connected to thef common terminali: connector 45* as indicatedat 95u', 97o andST-c.; However; inea'chfof those instances the other electr-'ode` is connected to a different part oi"rv tli current dividing circuit on the" opposite sid of theibatteries andY charging unit".v Inione instance theV electrode is: connectedto 'the terhiirial"- post 5B als'idic'ated at'QS'ai In another instanceitis connected to'fth'e midi-tapoff the 'rheo stat as'i indicated angela;I While inE anotherV in'- stnce n isf connected te" the movable contacter SUonftherhe'ostatas'indicated att-98e.- Y

With any of the depicted corineetionslfor'the welding Y electrodes iti may be observed that both grups `of`batter-ies and: the battery charging unit eh contribute tothe total Welding2 current ald thoiighfthe proportionate amounts 'contributed v by each' are' different because'` of thedifferent Vin`- teivening resistance values. The total anion-rits of pwereontributed to thweicing" electrodes by each of` the batteries andthe battery charging' unit' Y are. also varied`= by Y the' differentv circuit conrtions of the elefctrodesas" a result of thedifference of the load-y cairying'characteristics of thev twoVT groups' off batteries. f` Forv instance, with the''eleotrodesconnected as indicated at 91ay and SBa' they"r are connected directly across' the' bat'h tery; 315, whuetn'e connection tothe batteries 3 5 and 355and' that to:tliebattery charging unitare through portions ofv the rhe'ostat which are dependent'uponthe'isetting ofthe movable contacter g across-the' parallel connected batteries 35 and 35 which' have a higher normal power outputI ca:- pacity.' Y ,.v. With the electrodes connecteda's indicatedJ -at `91T and 98o, lootlfifgroups of battcries"v contribute their proportionate amounts of'4 thewelding c'urrenty through the resistanceu legs of thefrheostat.

The connections of theYV electrodesv indicatednat '51C andig'c'provi'de a'conside'rable amount of ilexibility for' adjustment sin'ce' one of theele'ot-rodes is connected to the movable contact'or 58 of the meester.- Therver'iauons of thweloing ele@- tr'ode connections vihi'chVi are' possible with preferred current dividing circuit add consideriable` flexibility' to the-- adjustments which? may be accomplished by varying the charging cur-rent irater andf the position? of the movable contacter on the rneostat to suit me welding current-vamos and' characteristics to a wide variety of mate'- rials-andaveights of material..

The use ofv my disclcsed'power conversion unit' requires only the connection ofithe-outiut'terminals thereof to suitable Welding electrodes-,the connection of the batterycharging unit toa com- Ine'rcial alt'ernatingcurrent'source of power and the connection of the' water cooled'condu'ctors'lto afsourceV of1 water an'd a drain. Since therequired aniount'of .water is' smalLthe connections tothe Water cooled` conduotors may be' made through temporary hose connections" if desired.

Also; since thepower'supply line' contri-butes'l'only aismall proportion of the total*welding-current demand during any one Welding o'peratior'il and spreads: the? accumulation" of the* Welding power by' charging thebatteries over periods of timei the'- instantaneous loadl demands of' my l v power conversion unit are not such that they require special power supply connections asisj the oase' with many types of! welding equipment. The p'referred rectifier characteristics for use-in the disL closed circuitV provide" a fail-ly uniforinfontput wattage atk rated output voltag-:lr and current While' charging the batteries, Y andA a lowervolt'age but: correspondingly higher output current while supplying current toI the output load.- This p'rovidesvl a load` factor'v and maximum' current demands'whi'ch arev satisfactory in mostv anyy locattion. Furthermore; Asince my'po'wer conversion unit supplies direct current to the welding elec'- trodes;l the Welding elicien'cy' of the'powe'ro'utput of my power conversion unit is high.

rThe water cooling of the battery connecting leadsinot. only' maintains the resistanc'es of those leads' at low Values, but also has altendenc'y to lkeep the battery plates cooled by the heat con'- duction through the terminal' posts. By my preferred system of connections-'for the batteries and the rl'xeos'tat,V the coolingof both the battery leads and the rheostat is effected by a-single continuous now of the cooling water. Y

.Inf the. modi-ned forms of my invention-*depicted in Figs'. 6 and. 7', reference numerals similarto those' previously' used designate corresponding parts. The illustrated variations of thelcircuit shown in Fig. 6 center around differences of the rheostat. connections' and resultingv changes inl its operation. Fig. 7' provides an example of structure which may be utilized for carrying out the circuit changes of Fig. 6 without foregoing the cooling of the rheostat legs by a' flow of coolant.

As will be observedfroln acomparisonjvr of Figs. 5 and 6, the rheostat legs in Fig; 5 have"a`=mid tap connector 56 while in the circuitl of Fig. 6,th'e resistance legs 52 and 55 of the rheostat 53 are ilrgialy connected only by the' movable connecyor Y In the modified circuit shown'in Fig. 6; one side lof the battery charging unit 25 is connectedV to an `end terminal E55 of the resistance 1e'g52. There being nov mid-tap connectionbetween the ends of the resistance legs 52 and 55, asiin Fig.

.5; the output loadl or welding: electrodes P06 and .ing the advantages of the Water cooling of the resistance legs 52 and 55 ofthe rheostat;

For purposes such as that aforementioned, an

electrically insulating` luid'low connection is provided between the ends of resistance legsv 52 and 55 by a tubular insulating memberl |09? made of a material such as Micarta and a T-'c'oup'ung Ht ononeend of theresistanee leg 5 5" l VWhile I have illustrated and described? av preformed embodiment of my'invent'ionmany modi'- cationsf may be: made without departing freni C: the Spirit of the invention, and I- d not lisl. tb

be limited to the precise details of construction Set forth, but desire to avail myself of all changes within the scope of the appended claims.

Having thus described my invention, what I claim as'new and desire to secure by Letters Patent of the United States,is:

1. In a power conversion system and apparatus, a unitary power conversion assembly adapted to bev connected to a source Vof power for supplying high current at low voltage to a load and coinprising, in combination, a support structure carried by casters and including a base and a shelf connected by support posts; a plurality of storage batteries of equal voltage rating carried by said shelf and each having the cells thereof connected in series; a battery charging unit Vmounted on base and adapted to be connected to said source of power; a rheostat carried by said frame and including a mid-tap, resistance legs extending from the mid-tap and a movable contact element providing a variable connection between said re sistance legs; said batteries, battery charging unit and rheostat being connected in a current dividing circuit wherein terminals oi one polarity on said batteries and battery charging unit are connected together; said batteries being unequaily divided with the majority thereof connected :in parallel and to one extending leg of the rheostat; the remainder of said batteries being connected to the other extending leg of the rheostat; a secyond output terminal of the battery charging unit being connected to said mid-tap of the rheostat so that charging current flows to the batteries through said legs of the rheostat; and rheostat and the common connection of said batteries and battery charging unit providing output connections for the power conversion assembly.

2. In a power conversion system and apparatus, a power conversion unit adapted to be connected to a source of power for supplying high current at low voltage to a load and comprising, in combination, a unitary enclosure having mounted therein a plurality of storage batteries of equal voltage rating, a battery charging unit adapted to be connected to said source of power, and a rheostat including resistance legs extending from a portion providing a mid-tap and a movable contact element providing a variable connection between the resistance legs; said batteries, battery charging unit and rheostat being 'electrically connected in a current dividing circuit which determines the current provided to the load by each of the batteries and the battery charging unit and wherein terminals of like polarity on each of said batteries and said battery charging unit are connected together, a part of said batteries being connected in parallel and to the end of one resistance leg of the rheostat, the remainder of said batteries being connected to the end of the other resistance leg of the rheostat, said battery charging unit having a second output terminal connected to the mid-tap of the rheostat so that charging current flows to the batteries through said resistance legs, and means electrically connected to said rheostat and to said connected terminals of the battery charging unit and batteries for providing output terminals for the power conversion assembly.

3. 1in a power conversion system and apparatus, a power conversion unit adapted to be connected to a source of power for supplying high current low Voltage to a load and comprising, in combination, a current dividing circuit having three branches, one of said branches including a rheostat having two resistance legs connected together at a mid-tap and a battery charging unit having Tij output terminals,one of which output terminals is connected to said mid-tap, said battery charging unit having input leads adapted to be connected to said source of power, a second of said branches including a secondary battery of prede termined voltage connected across the output terminals of the battery charging unit through one o1" said resistance'legs of the rheostat, the third branch of said circuit including a plurality of secondary batteries of said predetermined voltage connected in parallel and the said parallel connected batteries being connected across the output terminals of the battery charging unit through the other resistance leg of said rheostat, a connector movable along said resistance legs for varying the effective resistances thereof, and load current supply leads connected to said one of the branches of the current dividing circuit on opposite sides of the battery charging unit.

4. The combination donned by claim 3, and wherein one of the load current supply leads is connected to one of said resistance legs of the rheostat.

5. The combination defined by claim 3, and wherein one of the load current supply leads is connected to said connector.

6. The combination defined by claim 3, and wherein one of the load current supply leads is 'connected to said mid-tap on the rheostat.

lductors to provide a passage for the flow of cooling uid.

8. In a power conversion unit for an electrical system and apparatus and which is adapted to be connected to an alternating current power source and to furnish direct current to a load, the combination comprising a current dividing circuit in cluding three branches, one of said branches comprising a rheostat having two resistance legs connected together at a mid-tap and a battery charging unit having output terminals, one of which output terminals is connected to said mid-tap, said battery charging unit also having input leads adapted to be connected to the alternating current power source, a second of said branches including a secondary battery connected to the output terminals of said battery charging unit through one of said resistance legs of the rheostat, the third of said branches including a secondary battery connected across the output terminals of said battery charging unit through the other resistance leg of the rheostat, and a movable connector for varying the effective resistances of said resistance legs of the rheostat.

9. The combination dened in claim 8, and wherein said secondary batteries having the same voltage ratings and diilerent lo-ad carrying ca pacities, and means providing load current terminals connected to said current dividing circuit so as to receive current from both batteries and said battery charging unit.

l0. rThe combination dened in claim 8, and wherein means providing load current terminals connected to said current dividing circuit so as to receive current from both batteries and said battery charging unit, and said movable connector providing an adjustment for varying the proportions of the load current provided by each of said batteries and the battery charging unit.

Y therethrough.

`13. In a power conversion unit adaptedto be connected to an. alternating current power source and to-furnish'direct.current at low voltage to a load, the combination-,comprising a plurality of .storage batteries anda battery charging unit, saidbattery .charging unit having rone output terminal connected to each of the storage batteriesl to provide 4,one outputconnection for the power conversion unit, arheostat, Saidbattery charging unit having anotheroutput terminal connected to the storage batteries through said .rheostat and means including said rheostat -ior providing a second output connection Yfor .the power conversion unit and for regulating the current output of thepower conversion unit.

14. In a power conversion lunit `for a welding system and apparatus and which .is adapted to beconnected to an alternating current source and to provide directV current for welding, the combinationcomprising aplurality of secondary battery cellssaid cells being divided betweentwo branchesof a.circuit, means .for connecting said cells .into groups with -the cells of each group havingsubstantially .equal voltage ratings and unequal power delivering capacities, saidgroups being connected in parallel circuit relation througha .resistance |unit,.means including said resistanceunit for providing, current output terminals for thebatteries, vand means forcharrgingrthe batteries.

15. The combination set forthin claim 14 and wherein said resistance unit is 'provided .with variablemeans for adjusting the relative output currents of the groups of cells.

16. vIn a power conversion unit adapted to be connected to an alternating current source and xtoprovide direct `'current to: a load, thelcombina= tion Vcomprisingl a pluralityl of secondary-battery cells, said cells'beingdivided'intoggroups, means 'for-connecting the cells of one'group intoseries circuit relationship toprovide l1a battery of lpredetermined rated voltage, means '.for connecting the cells of the othergroup vinto series parallel relationship to provide a battery of substantially thesame predetermined voltage, said means .in each instance comprising tubular conductors, and means including non-.conductive -tubes connecting said tubular conductors to provide a substantially continuous conduit for .the ow of -a cooling liquid .through the-tubular conductors.

.17. The combination denedinclaim 13,..and wherein said r'heostatcomprehendsa pairof .resistance legs, and a common movable connector connecting said resistance legs and simultaneously .varying itspositions of connectionsithereto.

EDWIN S, .TRUE.

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